Acoustic charge transport (ACT) phenomena in III-IV semiconductor material has only recently been demonstrated. Such devices have applications at high speed analog signal processors. Delay lines have been fabricated in gallium arsenide (GaAs) substrates comprising a surface acoustic wave (SAW) transducer that launches a surface acoustic wave along an upper layer of the GaAs substrate having transport channel formed therein. An input ohmic electrode sources charge to be transported by the propagating potential wells and a Schottky control electrode receives a signal for modulating that charge. Spaced down the transport channel are one or more nondestructive sensing electrodes for sensing the propagating charge and finally an ohmic output electrode for removing the charge.
Initial acoustic charge transport devices comprised a thick epilayer with vertical charge confinement accomplished by means of an electrostatic DC potential applied to metal field plates on the top and bottom surfaces of the GaAs substrate. The field plate potentials are adjusted to fully deplete the epilayer and produce a potential maximum near the midpoint thereof. Consequently, any charge injected into the channel is confined to the potential minimum.
Lateral charge confinement (Y direction) has been achieved in several ways. Typically, a mesa is formed to define a charge transport channel. However, for thick epilayer acoustic transport devices, the mesa must be several microns in height, a fact which presents problems in fabrication and is a major impediment to the propagating surface acoustic wave. Blocking potentials extending down both sides of the delay line have also been used to define the transverse extent of the channel, as has proton bombardment to render the material surrounding the channel semi-insulating.
Recently a heterostructure acoustic charge transport (HACT) device has been fabricated using a GaAs/AlGaAs heterostructure that is similar to that of quantum well lasers and heterostructure field effect transistors (FET). A HACT device vertically confines mobile carriers through the placement of potential steps that result from band structure discontinuities. Besides providing for inherent vertical charge confinement, the HACT devices are thin film devices whose layers have a total thickness of approximately 0.25 microns, excluding a buffer layer.
Known HACT devices provide only for electrical modulation of the charge propagating with the surface acoustic wave. It would be advantageous to have a heterostructure acoustic charge transport device which is capable of charge modulation by an input signal comprised of an optical beam. The present invention is drawn towards such a device.